System and method to keep continuity of media flows for a collaborative session without constant controller(s) involvement

ABSTRACT

A system of managing collaborative session control when controller is lost or changes to passive-control mode in a data communication network comprising of a control-capable terminal device that can generate and send user preferences on successive controller selection and control policy, a master server device that can make control transfer decisions or take over control upon events, a normal terminal device that can process and response to master server queries on control capacity and its willingness to take over control. These apparatus are connected to each other inside one collaborative session, regardless of their subscriptions. A method of control management of the collaborative session without constant enrollment of controller comprises the steps of sending different types of preferences to master server device; making control transfer or handover decision; and interacting with affected terminals.

TECHNICAL FIELD

The present invention pertains to the field of telecommunication in apacket-switched communications network. More particularly, it concernssession control in an IP Multimedia System.

BACKGROUND ART

As more and more devices gain networking capabilities, the need for auser to manage these multiple devices arises. Such a work item has beenundertaken in 3GPP under the scope of collaborative session management.Here, a number of user's devices that registered with IMS services cancooperate with each other in a session with different media flows. Acollaborative session is a multimedia session with multiple UEs involvedand collaborative with each other. Controller UE manages media oncontrollers by interacting with an Application Server. Any request fromcontroller must be authorized by controller before taking into action.Usually one media flow is controlled by just one controller.

Due to the single controller configuration (for a specific flow) in acollaborative session, there exists a problem when controller is lostdue to uncontrollable reasons like UE breakdown, battery exhausted, UEout of coverage, unstable signal, etc. There are also situations whencontroller wants to change itself to passive-control mode or leavecollaborative session temporarily. These cases may happen whencontroller doesn't want to be interrupted each time a controller makes achange, or when no controller propose any request for a long time. Herepassive-control mode means that the controller UE opts to be inauto-control mode or temporarily gives the control to ApplicationServer. That is, the controller UE stays passive by setting up rulesthat make decisions on certain trigger scenarios or assigningresponsibility to other nodes such as Application Server or ControllerUEs.

In latest 3GPP TS23.237, it is indicated that SCC AS releases all AccessLegs participated in a Collaborative Session when controller is lost.

The problems of this approach are that the controllers are always forcedto terminate the session without knowing what has happened, and theycannot continue or resume even if the user wants to continue and iswilling to pay.

Another possible approach is to allow the SCC AS to transfer theCollaborative Session control to another UE involved in theCollaborative Session and belonging to the same subscription”, if thecontroller is lost [Non-patent document 4].

This method makes a step to solve this problem but it doesn't specifyhow to select the successive controller and what happens if other UEsare under different subscription. Obviously, some better solutions areneeded to solve the problem for controller wants to change topassive-control mode, which is inevitable when the operator deployed thecollaborative session service.

CITATION LIST Non Patent Literature

[NPL 1]

-   3GPP TS 23.237 v9.2.0, “IP Multimedia Subsystem (IMS) Service    Continuity”

[NPL 2]

-   3GPP TS 24.237 v9.0.0, “IP Multimedia Subsystem (IMS) Service    Continuity”

[NPL 3]

-   3GPP TR 23.838 v9.0.0, “IP Multimedia Subsystem (IMS) service    continuity enhancements; Service, policy and interaction”

[NPL 4]

-   3GPP TSG SA WG2 Meeting #76, 16-20 Nov., 2009, San Jose Del Cabo,    Mexico TD S2-096767, “Requirement of Control transfer upon lost of    Collaborative Session control”

SUMMARY OF INVENTION

It is an objective of the invention to address the above mentionedproblems, shortcomings, and incompleteness. In particular it aims toprovide a method to support continuity of the collaborative session whencontroller is not available.

Another objective of the invention is to provide a robust systemtolerating to controller loss, containing Application Server and UEswithout limitation on releases, subscriptions and capabilities. In thesystem, a collaborative session is established with single or multiplecontroller(s). Each controller has its own responsibility to controlcertain media flows. All terminal UEs cooperate with each other and withthe Application Server to avoid session interrupt when controller islost by accident or controller leaves.

In one aspect, control-preference information is sent to an applicationserver. When controller is lost or changed to passive mode, a newcontroller could be decided by referring to the reference or decide bydefault rules. Subsequent action may be transferring the control toanother device, which will be asked for willingness to take over thecontrol and charge.

In another aspect, preference contains one or more lists. These listsare used to designate the successor controller if current controller islost; to provide rules on how to choose its successor; to set limitationon media management; and to set trigger point for session release.

In yet another aspect, the terminal is capable of requesting theApplication Server to change control to passive control mode. Duringpassive control, normal decisions like media resolution modification etcare automatically made by Application Server through preference controlrules set at the beginning of a session or IMS registration, If queryfrom Application Server is received, the terminal has a function toprocess and reply the query. Even if it cannot understand the query, thefunction still replies with an “unknown” message. These additionalfunctions extend the session control to auto-control and emergencycases.

In another aspect, the Application Server contains the preferenceprocessing function that can identify different type of preferences andprocess it for future use. It also has controller loss detectionfunction to detect the controller lost. It further has control transferdecision function refers to preferences when controller is notresponsive for some time.

In another aspect, control is extended to both authorization of thesession and the charge of media flows. Controller is responsible to makedecision on changes of the media flows it controls and it is also theentity that will be charged for those media flows it controls. Thepreference from controller will indicate how to relocate charging entityin case controller is lost. Control transfer and charge transfer areseparate decisions, but controller and Application Server may choose tocombine them in the preference and decision.

With these solutions, session has greater chance to continue whencontroller leaves. Both controller and SCC AS could participate in thedecision of control transfer. Subscription is not a limitation any more.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1]

FIG. 1 is a diagram illustrating the overall system. It consists ofseveral UE terminals with or without session control capacity, e.g. 101and 105; an application server (102) that coordinates session terminals;IMS Core Network (103) providing IMS signaling support; and remote party(104) that has session with the UEs.

[FIG. 2]

FIG. 2 is the structure of Terminal Devices that specifically perform ascontroller (101) in a collaborative session.

[FIG. 3]

FIG. 3 is the structure of Application Server that manages the wholesession.

[FIG. 4]

FIG. 4 is the structure of Terminal Devices (105) that perform as otheruser equipments in the collaborative session, controllers orcontrollers.

[FIG. 5]

FIG. 5 is a flowchart diagram illustrating how the Application Servermakes decision on control transfer when controller lost without noticeor controller changes to passive mode.

[FIG. 6]

FIG. 6 is a diagram illustrating the structure of preference that isgenerated by controller and stored in Application server. In the tree,different types of rules are demonstrated.

[FIG. 7]

FIG. 7 is a diagram illustrating an example operation sequence forcontroller changing to passive control mode with signals exchanges amongUEs and Application Servers.

[FIG. 8]

FIG. 8 is a diagram illustrating an alternative operation sequence forcontroller lost solution with controller set preferences on when torelease the session after controller lost.

[FIG. 9]

FIG. 9 is a diagram illustrating another alternative operation sequencefor controller lost solution with controller nominates its successor orset preferences on how to choose the successive controller.

[FIG. 10]

FIG. 10 is a diagram illustrating the a different operation sequence forcontroller lost solution with Application Server broadcast to UEs amongthe session about their capacity and willingness to take over control.

[FIG. 11]

FIG. 11 is a diagram illustrating another operation sequence forcontroller lost solution with Application Server queries on affected UEto take over charge of media terminates at it due to controller loss.

DESCRIPTION OF EMBODIMENTS

In the following detailed description of exemplary embodiments of theinvention, reference is made to the accompanied drawings, which form apart hereof, and which is shown by way of illustration, specificexemplary embodiments of which the invention may be practiced. Eachembodiment is described in sufficient detail to enable those skilled inthe art to practice the invention, and it is to be understood that theembodiments may be utilized, and other changes may be made, withoutdeparting from the spirit or scope of the present invention. Thefollowing detailed description is, therefore, not to be taken in alimiting sense and the scope of the present invention is defined only bythe appended claims. In the following description, for the purpose ofexplanation, specific numbers, times, structures, protocols, and otherparameters are set forth in order to provide a thorough understanding ofthe present invention. However, it will be apparent to anyone skilled inthe art that the present invention may be practiced without thesespecific details.

FIG. 1 illustrates a system that supports the present invention,consisting of a Controller UE 101 that controls a collaborative session,a normal UE 105 that participates in the session without the right ofcontrol, an Application Server 102 that coordinates the session amongUEs and the remote party, the IMS Core Network 103 that provide IMSsignaling and routing functions for the session, and the remote party104 that has the session with the UEs. All UEs, controller orcontroller, communicate to Application Server through standard IMSprocedure. In the invention, communication from Controller toApplication Server 110 has additional information beyond standardelements and includes user preference for session control; and thecommunication from normal UE to Application Server 113 has additionalinformation beyond standard elements and includes UE capacityparameters. Only controller UE 101 is required to send controlpreference; while other users 105 may choose to send or not send theircapacity parameters. The user control preference from 101 may be sent atcollaborative/interactive session setup or at IMS registration. Thecontrol preference could be in any format that is understandable by theApplication Server 102. It is used to instruct the Application Server onhow to perform control if the controller leaves with notice, and how tomanage the session if controller lost connection without notice. UEcapacity parameters could be sent in any format that is understandableby Application Server. It includes for example UE's capacity of control,battery level, IMS release version, etc. This additional informationwill be used by Application Server for decision making when controllerleaves the session or loses the connection. In the present invention,Application Server 102 has additional capability of taking over sessioncontrol based on control preferences and deciding control transfer toUEs belonging to the session. However, Application Server 102 will nevertake over charge for the session. Thus it will control the sessionrepresenting controller in authorization but never representing thecontroller in charging. Charging is required to be assigned to UE(s).Connection between Application Server and IMS CN 111 and connectionbetween IMS CN and Remote Party 112 make use of standard IMS proceduresand carry standard information defined in IMS.

FIG. 2, is a communication device with control capability. It can serveas the controller of a session. Besides traditional functions of userequipment, it also contains GUI block 201 for preference generatinginteraction; a User Preference Generator 202 connected to GUI; a UserPreference Transmission Function 204 that sends the preference throughTransmission Layer Functions 205 if it is the controller of the session;a Passive Control Function 203 that can initiate signals requesting tochange to passive control mode. The preference contains one or morelists. These lists are used to designate the successor controller ifcurrent controller lost connection; to provide rules on how to chooseits successor; to set limitation on media management; and to set triggerpoint for session release. For example, the use may indicate inpreference that “Session Termination: 10 min”. Then if it leaves, thesession will be released after 10 min from his left. Another example maycontain media management rules like “Add media: Reject; Modify media:Agree”. When controller leaves with this preference, Application Serverwill reject all add-media requests from controllers and agree allmodify-media requests.

Another new function for the terminal is to send request to ApplicationServer for changing itself to passive control mode. During passivecontrol, normal decisions like media resolution modification etc areautomatically made by Application Server through preference controlrules set at the beginning of a session or IMS registration, or updatedusing any IMS procedures. To generate user preference, User PreferenceGenerator 202 prepares questions and queries to user through GUI 201.User's answer of the question is stored and processed at User PreferenceGenerator 202, from where a preference file is generated in the formatthat is understandable by Application Server 102. It is obvious toanyone skilled in the art that this preference file also can be loadedinto the terminal via different means, e.g. a storage card, download viainternet, transferred via Bluetooth from another terminal, etc.

When a terminal registers as a controller, User Preference TransmissionFunction 204 is triggered to send the preference out. The preference istargeted at solving the control handover problem in case the controllerloses connection. The preference can also contain a set of rules toperform automatic control when controller intentionally changes topassive-control mode. For terminals that are not a controller, UserPreference Generator 202 can skip the procedure of generating a userpreference during registration. It is obvious to anyone skilled in theart that the preference can be generated later at any time, before theterminal becomes the controller. Additionally, the preference can beupdated during the session when changes happened to the session.

FIG. 3 illustrated an example structure for Application Server 102 thatmanages the collaborative session. New functionalities are introduced tothe Application Server. It contains Preference Receive Function 301 thatfilters the control preference from other register information;Preference Process Function 303 that analyzes the received controlpreference; Controller Loss Detection Function 302 that periodicallychecks the availability of controller; Control Transfer DecisionFunction 304 that decides the control (and/or charge) transfer in caseof controller not involved, based on default rules stored in ApplicationServer or control preferences passed from Preference Process Function303; Passive Control Function 305 conducts control when controllerchanges to passive-control mode or session release procedure isactivated after controller's lost.

In cases when Application Server does not have capacity parameters ofother UEs in the session, it needs to query UE on such information fordecision. UE Query Function 306 is to fulfill this purpose. Afterobtaining enough information, Control Transfer Decision Function 304decides to perform control transfer or release the session.

If control needs to be taken over by Application Server, it willactivate Passive Control Function 305 to conduct control based on userpreferences. With these function, Application Server acts as anintelligent agent that can save the session by selecting andtransferring control to another UE or even take over control itself whencontroller is lost or left. Preference Process Function 303 isresponsible to explain and classify the control preferences written inany format agreed between terminal and Application Server. For example,the preference may be written with XML and it indicates that thecontroller successor can only be selected under the same subscription.After processing, this preference is passed to Control Transfer DecisionFunction 304. When Control Loss Detection Function 302 detects acontroller loss, through a timer or other bearer monitors, the ControlTransfer Decision Function 304 will only consider those terminals underthe same subscription as previous controller to be the successivecontroller. If no terminal UE is of the same subscription as the lostcontroller, Application Server should treat it as no preference case.Other operation sequences of the present invention for that case can beutilized to handle it.

FIG. 4 illustrates an example architecture for a Terminal Device 400that is a normal communication device with or without controlcapability. It acts as controller or controller in the collaborativesession, but it is not the target controller which will lose connectionor changes to passive control mode. Besides traditional functions ofnormal user equipment, it also contains additional function block thatcan process and response the query from Application Server 102. In thisinvention, Application Server may query terminals for their controlcapability and willingness to take over the control and charge. QueryReceive Function 401 and Query Process Function 402 are used to receivesuch messages and process them. The processed query will be passed toQuery Response Function 403 to generate response back to the ApplicationServer 102.

UE Configuration/Status Record Function 404 serves like a database. Itprovides the parameters and status of the UE and assists Query ResponseFunction 403 to generate the response to Application Server.

If the query cannot be understood by the Query Process Function 402,Query Response Function 403 would generate a response to ApplicationServer 102 indicating that it received an unknown query.

All signaling messages exchanged among 200, 300, 400 can be transportedover normal IMS mechanism, e.g. via TCP channel or UDP channel withretransmission mechanism. User preferences are sent together with SIPsignal during IMS registration or in a separate packet duringcollaborative session establishment, or when a UE becomes a controllerUE. User decides how many preferences to generate through GUI onTerminal Device 200. All generated preferences will be sent toApplication Server 300 from a controller UE.

FIG. 5 is the flowchart of example logic for the Application Server 102,which is a major management and decision making device. Solutions forcontroller loss or passive control problems are summarized in thisflowchart.

The diagram consists of major two branches. One is the situation thatApplication Server needs to take over the control. The other isApplication Server does not need to take over the control. The secondcase is further divided into two braches. One is that controllerpreference is available and feasible to make decision. The other ispreference is not available or exist preference cannot be applied due toconfliction with current situation.

When Application Server 102 detected a controller loss or receives asignal indicating a controller changes to passive-control mode, itperform step 502 to check whether a corresponding user preference isavailable. In case where preference is available, it continues to step503 and checks whether it needs to take over the control.

There are two situations where Application Server 102 needs to take overcontrol. One situation is controller changes to passive-control mode andrequests Application Server to answer control related questions insteadof processing it on controller. The other situation is that controllerlost connection without notice, and according to pre-set preference, theApplication Server 102 is responsible to handle the session, e.g.release it after some trigger, select a different controller andtransfer the control over, etc.

In case that Application Server does not need to take over control, itwill further go to step 505 to check current session status parameters.Step 506 is a checking procedure that matches user preference withcurrent session status. Current session status includes all informationrelated to the current session. For example, the ID of UE involved inthe session, the subscription of each UE, the number of media terminateat each UE, etc. Matching preference with session status is to comparethe string or value from two parts. For example, if preference indicatesTom is the successor, Tom will be translated to Tom's UE's ID byfunction 303 and step 506 will compare this ID with all participated UEIDs in the session. If preference indicates the UE that has maximumnumber of media flows taking over the control, step 506 will check ifthere is a UE that possesses maximum number of media flows. If UEsatisfying the preference criteria exists in the session, it is saidthat current session status matches the user preference.

If current session status mismatches with user preference, the decisionmaking procedure will be directed to step 511, which is a branch whereno preference is available. An example of mismatch is as following: Userpreference indicates John will be the successor of current controller.However, when controller is lost, John already left the session. Thisexample can be avoided if Application Server can send a trigger toController UE to update its control preference whenever somethingchanges in the session. However, without such kind of triggers, mismatchmay happen.

If no mismatch happens between current session status and userpreference, in step 507 a successor is decided and selected successor isasked in step 508 whether it accepts to be a new controller. If theselected terminal (successor) is capable of control and agreed to takeover, the control is transferred to it; while if it rejected the offeror not capable of performing the control, other operations will be takenin step 510, e.g. release the session. The selection-query-responseprocedure may be repeated before session release if multiple terminalssatisfy the user preference criteria.

When controller is lost without preference, Application Server 102 canneither decide to transfer control nor take over control. It can onlytry to save the affected session at step 512 by checking whetheraffected users are willing to take over the charge of his media flow(s).If yes, charge will be transferred to affected user in step 513; whileif no, both control and media will be released in step 510. Note that ifaffected user is the last UE in the collaborative session, there will beno collaborative session anymore after charge transfer, The affecteduser changes to a normal IMS session, continuing his media flow withremote party 104.

FIG. 6 illustrates the structure of preferences. Six sets of rules arepresented based on solutions in this patent. List of priority ofsuccessive controllers 551 is a set of rules that designate successivecontrollers using exclusive identities like subscription information562, user name 563, UE Identification Number 561, etc. It also specifiesthe priority of these potential successors so that Application Serverknows who to choose first when controller is lost. For example, thepreference indicates the successor sequence is Tom-Marry-John. ThenApplication Server will ask Tom first to take over control in case ofcontroller loss. If Tom rejects the request, Marry will be asked. Such apreference will be updated whenever UEs join or leave the session.

If controller does not want to explicitly designate its successors,Successive Controller Selection Rules 552 may be used to set criteriafor Application Server to select the successor. For example, successorsare selected according to the sequence of controllers join the session568. The Application Server will record the join sequence of controllersand select based on it. Another criterion is UE capacity 571, UEcapacity includes UE's ability to control, UE's battery level, UE'ssignal stability, etc. Rule 559 set the criteria of choosing successivecontroller to be the same subscription. In this case, Application Servermay refer to default rule 590 to choose successive controller withinthose UEs under the same subscription as lost controller.

Termination Rules 553 is a rule set that determines trigger events toterminate a session after loss of controller. It may set a time out 575for the current session; it may limit the Bytes consumed by controllers576; it may terminate only certain type of media 577 (e.g. video flows596); it may set maximum amount of money chargeable 578 after thecontroller is lost.

Media Management Rules 554 are specially used for passive control mode.Application Server could make control decision based on these rulesrepresenting the controller. Charge Transfer Rules 555 decide whethertransfer charge together with control or separate from it. If separatingfrom control transfer, the preference will give explicit rules 597 tospecify who to transfer the charge. These rules may be similar to rulesof successive controller selection but they need to be executedseparately from control transfer when controller is lost.

Default Rules 556 are stored at Application Server and serve as backuprules when controller preference does not give a specific candidate forcontrol/charge transfer or when controller preference does not give aspecific answer for a controller request. For example, a controllerpreference only specifies that successive controller should be selectedwithin UEs under the same subscription 559. Then Application Server willuse default rule 590 to choose a unique candidate. Another example isthat controller request to change one component of a media flow butcontroller did not give rules for this request in Media Management Rules554 before changing to passive control mode. In this case ApplicationServer will use default rule 591 to reject the request representing thecontroller.

FIG. 7 illustrates an example operation sequence of the presentedsolution. It illustrates the solution when controller changes to passivecontrol mode. This solution contains Controller 601, Controller 602,Application Server 603, and Remote Party 604.

In step 610, a collaborative session is established with preferences.Application Server 603 processes the preference as in step 611. WhenController 601 requests to add media to Controller at step 612,Application Server accepts the request and performs the media adding atstep 613. After these steps, a collaborative session is activated withcontrol from Controller (614) and media flow between Controller andRemote Party (615).

Then Controller requests to change to passive control mode at step 616.Application Server returns an acknowledgement and loads user preferenceon step 617. After successful loading the preference, the collaborativesession changes to passive control mode (step 621). Under passivecontrol mode, if any request comes from the Controller (619),Application Server 603 will look up for control rules in preference(620), make decision and perform the decision (621). After a while,Controller 601 can request to change back to active control mode (623).At the point of receiving this message, Application Server needs todeactivate its Passive Control Function 605 and return to normal mode(625).

FIG. 8 illustrates another operation sequence of the present solution.It illustrates the solution where controller set criteria on terminatingthe session upon his lost. This solution contains Controller 651,Controller 652, Application Server 653, and Remote Party 654.

At the beginning of the session, preference is sent and processed atApplication Server in steps 660 to 661. This preference does not containrules to select a successive controller, but it contains the criteriaabout when to terminate the session if controller is lost. For example,it specifies a timer that should start from the detection of it lost.When the timer expires, the whole session is torn down.

In step 663, the Application Server (653) detects controller loss, andit will automatically start session termination control by PassiveControl Function 305. The Application Server may send a signal toaffected controller(s) to inform them their session will be terminatedafter some time, as in step 665. This signal will help controller tocomplete the most important conversation before their session isforcefully released. When termination event happened as preferenceindicated (666), the whole session is terminated in step 667 and 668.

FIG. 9 illustrates another operation sequence of the present inventionwhere the controller nominates its successor or set criteria on how toselect it successor if it lost connection. This solution containsController 701, UE-1 702 and UE-2 703, Application Server 704, andRemote Party 705. UE-1 and UE-2 may be other controller(s) in thesession, or they may be controller(s).

In this solution, Application Server obtains preference after steps 710and 711. When the Application Server (704) detects a controller loss asin step 712, it loads preference as in step 713 and matches thedesignated successor or successor selection criteria with current statusin step 714.

The selected UE will be queried on his willingness to take over thecontrol as in step 715. If selected UE accepts the request, control istransferred to this UE as in step 720. If selected UE rejects therequest, actions will be taken. For example in step 730, the wholesession will be released.

FIG. 10 illustrates yet another operation sequence of the presentsolution where no preference is set during IMS registration Orcollaborative session establishment. This solution contains Controller751, UE-1 752 and UE-2 753, Application Server 754, and Remote Party755. UE-1 and UE-2 may be other controller(s) in the session, or theymay be controller(s).

In this solution, when the Application Server (754) detects a controllerloss, no preference is provided in step 760. To save the on-going mediaflows, the Application Server broadcast requests to UEs with controlcapacity at step 762 querying their willingness to take over thecontrol. The first UE accepted the request (as in step 763) will beselected as new controller. If nobody wants to take over the control,the session will be released after certain time.

FIG. 11 illustrate another operation of the solution when there is nopreference set by controller before the session. It focuses on theterminal that affected by the loss of a controller. This solutioncontains Controller-1 801, Controller-2 802, UE 803, Application Server804, and Remote Party 805. Controller-1 controls Media-A and it is thecontroller that going to be lost, while controller-2 is anothercontroller in the session controls different media (Media-B). UE 803 isa controller that has Media-A with Remote Party 805.

When Application Server (804) detects Controller-1 801 is lost, it sendsquery 814 to affected UE 803 for charge transfer since Media-Aterminates at UE 803. If UE 803 accepts the transfer, steps in 820 areconducted and he will continue the media with Remote Party 805. If UE803 rejects the charge transfer, media flow may be cut as steps in 830.The Media-B controlled by Controller-2 802 will not be influenced.

The invention claimed is:
 1. A method for a system including anapplication server and UEs comprising: transmitting, from a controllerUE which controls a collaborative session among the application serverand the UEs including the controller UE, preference rules to be used ina special case where at least one of the controller UE is lost from thecollaborative session and a mode of the controller UE is changed to apassive control mode; selecting, in the special case, a single candidateof successor apparatus from among candidates of successor apparatusbased on the preference rules; requesting, the single candidate of thesuccessor apparatus to take over a control of the collaborative session;and requesting, when the single candidate of the successor apparatusrejects to take over the control of the collaborative session, anothersingle candidate of the successor apparatus, selected from among thecandidates of the successor apparatus based on the preference rules, totake over the control of the collaborative session, wherein: thepreference rules contain, at least, information indicating candidates ofthe successor apparatus.
 2. The method of claim 1, wherein, thecandidates of the successor apparatus include the UEs involved in thecollaborative session.
 3. The method of claim 2, wherein, the candidatesof the successor apparatus further include the application server. 4.The method of claim 1, wherein, when all candidates of the successorapparatus reject to take over the control, the collaborative session isreleased.
 5. The method of claim 1, wherein, default rules are used fordetermining whether or not to take over the control to the successorapparatus, when one of (a) at least one of the controller UE is lostfrom the collaborative session and the mode of the controller UE ischanged to a passive control mode, without transmitting the preferencerules, and (b) the transmitted preference rules are one of notapplicable and not sufficient.
 6. The method of claim 5, wherein, thedefault rules are stored at the application server and are notchangeable by any UE involved in the collaborative session.
 7. Themethod of claim 1, wherein, the preference rules are transmitted to theapplication server.
 8. The method of claim 2, wherein, the preferencerules are transmitted at one of a registration of IMS service and at aregistration of inter-UE transfer service.
 9. The method of claim 1,wherein, the controller UE is determined to be lost from thecollaborative session when at least one of the connection with thecontroller UE is lost and the controller UE has not been controlling thecollaborative session for a predetermined time period.
 10. The method ofclaim 1, wherein the preference rules further contain at least part ofthe following information: a set of rules to select the successorapparatus, a set of rules to make default control decisions, and a setof events that will trigger releasing the collaborative session releaseafter the controller UE has been lost.
 11. An application servercomprising: a receiver that receives, from a controller UE whichcontrols a collaborative session among the application server and UEsincluding the controller UE, preference rules to be used in a specialcase where at least one of the controller UE is lost from thecollaborative session and a mode of the controller UE is changed to apassive control mode; a selector that selects, in the special case, asingle candidates of a successor apparatus from among candidates of thesuccessor apparatus based on the preference rules; and a requester that:requests the single candidate of the successor apparatus to take over acontrol of the collaborative session, and requests, when the singlecandidate of the successor apparatus rejects to take over the control ofthe collaborative session, another single candidate of the successorapparatus, selected from among the candidates of the successor apparatusbased on the preference rules, to take over the control of thecollaborative session, wherein: the preference rules contain, at least,information indicating candidates of the successor apparatus.
 12. Theapplication server of claim 11, wherein, the candidates of the successorapparatus include the UEs involved in the collaborative session.
 13. Theapplication server of claim 12, wherein, the candidates of the successorapparatus further include the application server.
 14. The applicationserver of claim 11, wherein, when all candidates of the successorapparatus reject to take over the control, the collaborative session isreleased.
 15. The application server of claim 11, wherein, thedetermining section is configured to use default rules to determinewhether or not to take over the control to the successor apparatus, whenone of (a) at least one of the controller UE is lost from thecollaborative session and the mode of the controller UE is changed to apassive control mode, without transmitting the preference rules, and (b)the transmitted preference rules are not applicable or not sufficient.16. The application sever of claim 11, wherein, the default rules arestored at the application server and are not changeable by any UEinvolved in the collaborative session.
 17. The application server ofclaim 11, wherein, the receiving section is configured to receive thepreference rules at one of a registration of IMS service and aregistration of inter-UE transfer service.
 18. The application server ofclaim 11, wherein, the determining section is further configured todetermine that the controller UE is lost from the collaborative sessionwhen at least one of the connection with the controller UE is lost andthe controller UE has not been controlling the collaborative session fora predetermined time period.
 19. The application server of claim 11,wherein the preference rules further contain all or part of thefollowing information: a set of rules to select the successor apparatus,a set of rules to make default control decisions, and a set of eventsthat will trigger releasing the collaborative session release after thecontroller UE has been lost.
 20. A system performing a collaborativesession comprising: a controller UE that controls the collaborativesession among an application server and UEs including the controller UE,and transmits preference rules to be used in a special case where atleast one of the controller UE is lost from the collaborative sessionand a mode of the controller UE is changed to a passive control mode,and the application server that: selects, in the special case, a singlecandidate of the successor apparatus from among candidates of thesuccessor apparatus, based on the preference rules, requests the singlecandidate of the successor apparatus to take over a control of thecollaborative session, and requests, when the single candidate of thesuccessor apparatus rejects to take over the control of thecollaborative session, another single candidate of the successorapparatus, selected from among the candidates of the successor apparatusbased on the preference rules, to take over the control of thecollaborative session, wherein: the preference rules contain, at least,information indicating candidates of the successor apparatus.